\begin{table}%t2
\caption{\label{tab:Tab2}Linearity fit parameters and their quality ($b>1$ denotes super-linear and $b<1$ sub-linear behavior).}
%\centering
\par
\begin{tabular}{clll} 
\hline\hline             
ch.\# & detector & Measurement, $\lambda$ & Parameter $b$ ($\pm$uncertainty)\\
\hline
  \multicolumn{4}{l}{\it Unit 1} \\
%\hline
  1-1 & MSM12 & NI 121.6~nm & 1.018  (0.018)\\
  1-2 & PIN10   & NI 210~nm & 1.010  (0.002)\\
  1-3 & MSM11 & NI 50~nm  & 1.131  (0.046)\\
  1-3 & MSM11 & GI 18~nm  & 1.179  (0.007)\\
  1-4 & AXUV-20D & GI 10~nm & 1.018  (0.002)\\
\hline
  \multicolumn{4}{l}{\it Unit 2} \\
%\hline
  2-1 & MSM21 & NI 121.6~nm & 1.033  (0.010)\\
  2-2 & PIN11   & NI 210~nm & 1.006  (0.004)\\
  2-3 & MSM15 & NI 50~nm  & 1.022  (0.038)\\
  2-3 & MSM15 & GI 18~nm  & 1.090  (0.005)\\
  2-4 & MSM19 & GI 10~nm  & 1.086  (0.009)\\
\hline
  \multicolumn{4}{l}{\it Unit 3} \\
%\hline
  3-1 & AXUV-20A & NI 121.6~nm & 1.039  (0.010)\\
  3-2 & PIN12   & NI 210~nm & 0.989  (0.005)\\
  3-3 & AXUV-20B & NI 50~nm  & 0.978  (0.011)\\
  3-3 & AXUV-20B & GI 18~nm  & 1.001  (0.001)\\
  3-4 & AXUV-20C & GI 10~nm  & 0.998  (0.002)\\
\hline
\end{tabular}
\end{table}